1. Field of the Invention
This invention relates to photographic silver halide emulsions generally and more particularly relates to photographic silver halide emulsions having improved tolerance to bright room conditions.
2. Description of Related Art
Most conventional photographic materials are sensitive to visible light and, accordingly, must be handled under low intensity red, yellow or amber lighting, commonly referred to as "darkroom conditions," to prevent undesired exposure of the film. Also known in the art, however, are a type of photographic materials which can be handled under bright room conditions for a limited period of time without detrimental effects on the film. Such photographic materials are variously referred to in the art as "bright light films," "white light films," "daylight films," or "room light films."
Photographic materials which can be handled and processed in a bright room are specifically those which can be used in a room having illumination of at least 200 lux from a fluorescent lamp having reduced ultraviolet ray emission, or a filtered light not having the wavelength below about 420 nm as a safelight source. Typically, these photographic materials can be safely handled in bright room conditions on the order of 25 to 60 minutes, without detrimental effects on the film, such as fogging. To obtain such results, these photographic materials have greatly reduced sensitivity to visible light, that is about 1/1,000 to 1/10,000 that of ordinary darkroom photographic materials. Exposing these photographic materials requires the use of a high intensity light source rich in ultraviolet light, such as, for example a high pressure mercury lamp, a metal halide lamp, a microwave discharge type mercury non-electrode light source, and a xenon lamp. To produce the high intensity required to expose bright light films, the above-mentioned sources are broader than the point light sources used for conventional darkroom films and consequently emit more diffuse light. In order to obtain sharp character images, line images or dot images by contact exposure of such bright light silver halide photographic material with a broad high intensity source, the photographic material is required to have a high contrast photographic characteristic, i.e., a contrast value of 10 or more.
To obtain the desired high contrast, the emulsion for the photographic material typically contains silver halide grains having a high proportion of silver chloride, which have been chemically sensitized by sulfur, noble metal, or reduction sensitizers or combinations thereof. Sulfur sensitization is the most common chemical sensitizer used. However, chemical sensitization, particularly sulfur sensitization, extends the intrinsic light absorption of the photographic emulsion to light wavelengths longer than 420 nm, reducing the room light tolerance of the photographic material. Extension of the long wavelength limit of sensitivity produced by chemical sensitization is discussed in further detail in C. E. Kenneth Mees and T. H. James, The Theory of the Photographic Process, 3rd Edition, pages 113-116 (published by MacMillan Co., New York, 1966).
To compensate for this effect, yellow dyes having a peak absorption in the range of 400 to 550 nm are typically added to the photographic emulsion layer and/or to a layer above the emulsion to reduce sensitivity to light above 420 nm wavelength. However, dyes added for the purpose of enhancing the safelight tolerance of these photographic materials often reduce the contrast enhancement arising from chemical sensitization. Furthermore, the dyes affect light scattering properties within the photographic material in a way that reduces the capability to control line width of line images, commonly referred to as spread and choke, and to control the size of dot images, commonly referred to as dry dot etching.
Recently, the use of the lower wattage metal halide light sources and quartz iodide light sources, has become increasingly popular because of decreased cost, energy savings and improved convenience in use. These lower wattage light sources, however, have reduced ultraviolet light emission, which means that the bright light films must have a higher sensitivity for use with these light sources. In order to provide typical desired contact exposure times of about 5 to 15 seconds, photographic materials designed for use with these lower wattage light sources require about an order of magnitude higher ultraviolet light sensitivity compared to those designed for use with other high intensity light sources.
Furthermore, it is desirable to operate these lower energy light sources in a bright room, preferably, in a room where the illumination intensity is fully equivalent to the typical office environment (about 540 lux). To meet these seemingly contradictory needs, new photographic materials are required which have much higher sensitivity to ultraviolet light while simultaneously having much reduced sensitivity to light with wavelength greater than 420 nm.